Hi.. plastic bumpers require specialist products, and knowledge for a succesful job, best bet would be to get a decent bodyshop to do it for you!
If you really want to do it yourself, try this way, assuming that you are using aerosols..
and the bumper is "OFF" the car
wash the bumper with a strong detergent solution, and "Key" the area that you want to paintwith a peice of grey scotchbright abrasive pad, DO NOT use any type of rubbing paper, as this will rip the plastic to shreds!
Prime the bumper with the correct primer for the type of plastic (most bumpers are made from ABS) Halfords should sell this, once primed, leave for 10/15mins, then apply your basecoat paint, again allow 10/15mins to dry, before applying you lacquer
If you get it wrong, you could end up ruining your bumpers!
Tom